Asteel pipe of 3.5-in. outer diameter is to be used to transmit a torque of 2970 lb·ft without exceeding an allowable shearing stress of 8 ksi. a series of 3.5-in.-outer-diameter pipes is available for use. knowing that the wall thickness of the available pipes varies from 0.25 in. to 0.50 in. in 0.0625-in. increments, choose the lightest pipe that can be used.